There are many ways of creating round corner boxes on your site. The holy grail of these is to be able to accomplish the entire thing without the use of images and indeed a number of sites offer wizards to generate them.
To date I, and several other webmasters who have joined in the hunt with me, have failed to find one that works in all browsers and copes with text re-size without breaking.
If however you're willing to use 4 small images then the task is easily accomplished, in fact, I use it on this site.
And just to prove it works.........
here's one I made earlier
These are the 4 images you'll need right click on them to save them or, if you have a graphics package create your own. These are 14 pixels square.
If you copy and paste this make sure you paste it as plain text or you will get all the formatting.
background: #f90 url('bl.gif') no-repeat 0 100%;
background: url('br.gif') no-repeat 100% 100%;
background: url('tl.gif') no-repeat 0 0;
background: url('tr.gif') no-repeat 100% 0;
<!-- Start rounded box -->
The contents of the box go here
<br /><br />
<!-- End innerbox -->
<!-- End rounded box -->